<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>this</title>
</head>
<body>
  <!-- this始终指向调用它的对象 -->
  <!-- 1 -->
  var a  = {}
</body>
</html>